Newcastle disease virus (NDV) was used for inducing fusion of cells in preimplantation mouse embryos. In interspecies sperm-egg fusion, vitelline microvilli were lengthened by virus action and grasped the head of spermatozoa before the fusion of their plasma membranes. Virus-treated embryonic or limb bud cells were pipetted into 2-celled eggs, 20% of the embryos survived in cultures. When such cells were pipetted into 8-celled eggs, 15% of the 80% surviving embryos showed fusion of cells. Transfer of cell-injected 8-celled eggs to foster mothers gave birth of young. NDV was alone injected into blastocysts, viral replication was observed. After overnight culture some blastocysts appeared to contain some large round cells, which were thought to be fused cells. A markedly decreased number developed to fetuses in recipient females. Apparently, the preimplantation mouse embryos at 8-celled stage are good for the induction of fusion of cells in the embryos by NDV, and the embryos will contain a proportion of fused, or polyploid, cells for the study relating to the syndromes of abortion. We also expect to work on the inactivation and replacement of nuclear materials in mammalian eggs using argon laser microirradiation in conjunction with virus cell fusion method.